I had been a bit too generous with the PC creation rules, so I was compensating by boosting the opposition a bit. And then I checked the stats of the Ghost Lord and went "Really? CR 13 and topping out at 4th level spells (plus a 5th level spell already cast)? Nope, I'm gonna have to rewrite that Blighter* prestige class." And so I did, changing it so instead of having Blighter casting replace druid casting, it would add onto it like a normal PrC. And since I was already boosting everything, I gave him another level for good measure.
And of course, the Ghost Lord utterly massacred my poor PCs. The difference between one 11d6 flame strike on one hand, and finger of death and harm on the other, was quite pronounced. I think one of them managed to run away, but that pretty much ended the campaign for my group.
I still think my initial instinct that "There's no way that that's CR 13" was correct, but I definitely overcompensated. One of many examples of how 3e's rules for CR were, well, bad.
* Blighter was a prestige class for ex-druids turning to necromancy, sort of like the Blackguard for paladins, except they started over from 1st level spells but got one spell level per class level instead of per two as usual. So ex-druid 6/blighter 5 could cast 5th level spells, but in this case the 5th level spell was create undead which had already been cast off-screen.